The long-term goal of the proposed studies is to further characterize the effects on the central control of respiration of reflexes arising from the intercostal and abdominal respiratory muscles. The studies will be conducted on cats. The first objective is to determine if the inspiratory inhibitory action of intercostal and abdominal muscle afferents influences the activity of pneumotaxic center (nucleus parabrachialis medialis) neurons. Single pneumotaxic center neuron activity will be recorded while electrically stimulating selected intercostal nerves. The second objective is to determine if stimulation of primary muscle spindle endings in the external (inspiratory) and internal (expiratory) intercostal muscles, and the abdominal expiratory muscles can inhibit central inspiratory activity. Primary muscle spindle endings in the individual muscle types will be selectively stimulated by vibration, while recording medullary inspiratory neuron and phrenic activity. The third objective is to determine if the recruited units (non-inspiratory and expiratory) in the dorsal and ventral respiratory groups, that are elicited by stimulation of intercostal and abdominal muscle afferents, are inhibitory interneurons. Medullary inspiratory neuron recordings will be monitored for the presence of these recruited units during stimulation of the pneumotaxic center and pulmonary stretch receptor afferents, which are also inspiratory inhibitory. The proposed studies will contribute to the understanding of a) the medullary-pontine control mechanisms, b) the manner in which respiratory muscle afferent information is processed by the medullary-pontine mechanisms, and c) the muscle receptors whose afferents feed-back to the medullary-pontine areas.